Separation methods for water treatment include a heating or phase-change method, a filtration membrane method, and the like. According to the filtration membrane method, it is possible to obtain the water of quality as desired by controlling the size of the fine pores of the filtration membrane, which advantageously improves the reliability of process. Furthermore, since the filtration membrane method does not require a heating process, it can be advantageously used for water treatment using microorganisms that could be adversely affected by heat.
Among the filtration membrane methods is a method using a hollow fiber membrane module in which hollow fiber membranes are disposed in the form of a bundle. Conventionally, the hollow fiber membrane module is widely used in the field of precision filtration such as preparation of sterile water, drinking water, ultrapure water or the like. Recently, the application field of the hollow fiber membrane module extends to sewage/waste water disposal, separation of solids in sewage disposal tank, removal of suspended solids (SS) in industrial waste water, filtration of stream water, filtration of industrial water, filtration of pool water and the like.
The hollow fiber membrane module may be classified into a submerged module and a pressurized module depending on operation manner thereof.
The submerged module is immersed in a bath containing fluids to be purified. As a negative pressure is applied inside the hollow fiber membrane, only pure fluid is allowed to permeate the membrane and come into the hollow thereof while the contaminants such as impurities or sludge remains outside the membrane. The submerged module is advantageous in that it can decrease the costs for setting up the facilities and operating thereof since it does not require fluid circulation. It is disadvantageous, however, in that its permeation flux that can be obtained per unit time is relatively low.
On the other hand, in a pressurized module, as a positive pressure is applied to the fluid outside the hollow fiber membrane, only pure fluid is allowed to permeate the membrane and come into the hollow thereof while the contaminants remains outside the membrane. Although the pressurized module requires additional equipments for fluid circulation, it facilitates relatively high permeation flux per unit time, as compared to submerged module.
The pressurized module has a structure in which a bundle of hollow fiber membranes are potted in a module case having a predetermined size. For this reason, the packing density of the bundle of hollow fiber membranes is important in terms of a permeation flux. As the packing density increases, the whole area of the membranes increases, but, disadvantageously, the space in the module case for raw water to be filtered decreases and the membranes become more vulnerable to contamination. Also, when strong aeration cleaning is performed in order to prevent membrane contamination, the hollow fiber membrane may be impaired.
Accordingly, it is required that the pressurized filtration module of high permeation flux should have a superior durability so that the membranes could be cleaned without being impaired.